The identification of T cell activation and function genes (e.g., PTPN22, PTPN2, CD28, CD2, CTLA4, STAT4, PRKCQ, IL2RA, IL2RB) as risk alleles in genome-wide association studies [33] and the clinical efficacy of abatacept, a T cell co-stimulatory molecule/signal 2-targeted therapeutic [34] further strengthen the notion that RA is a T cell-mediated autoimmune disease. Here, PTPN22 is linked to autoimmune disease.